1. Technical Field of the Invention
This invention relates to resin-coated particulate solids. The invention further relates to a method of coating particulate solids with a curable resin. The invention still further relates to a method of treating a subsurface formation, such as a hydrocarbon containing subsurface formation, with particulate solids coated with a curable resin.
2. Description of the Prior Art and Problems Solved
It is known in the art to employ particulate solids in processes for treating subterranean formations. One such process is referred to as formation fracturing; another is referred to as gravel packing. The word proppant is employed in the art to describe a particulate solid employed in formation fracturing. Particulate solids employed in fracturing and gravel packing processes have been selected from various different materials including silica sand, sintered bauxite, glass spheres and ceramics.
Various publications, such as the ones identified below, disclose terms which are conventionally employed in the art to describe resins and their properties. Such terms include resin, fusible, infusible, thermosetting, cure and curing, thermoplastic, phenolics, phenol formaldehyde resin, novolac, resole, resite and epoxy resin. These terms, chemicals and related concepts are defined below and apply to the art in general as well as to the specific subject matter of this invention. Accordingly:
A resin can be a naturally occurring material or a synthetic material. A synthetic resin is a man-made polymer resulting from a chemical reaction between two or more substances, usually in the presence of supplied heat and/or a catalyst. It has been suggested, for purposes of precision, that the phrase “synthetic polymer” be employed instead of the phrase “synthetic resin” to emphasize the man-made nature of the material. The resins employed herein are preferably, but are not limited to, synthetic polymers.
The term fusible has been derived from the word fusion, which through custom of usage, has assumed the meaning of melt and blend. Accordingly, fusible is an adjective which clarifies that a designated material is capable of being softened, melted or liquified by heating.
The word infusible is the opposite of fusible. Accordingly, infusible is an adjective which clarifies that a designated material is unable to be softened, melted or liquified by heating.
In view of the above, a fusible resin, in the context of the present invention, is preferably a synthetic polymer capable of being softened, melted or liquified by heating, and, conversely, an infusible resin, in the context of the present invention, is preferably a synthetic polymer which, after having been cured, cannot be softened, melted or liquified by heating.
A thermosetting resin, that is, a thermoset, is preferably a synthetic polymer which, by application of heat, by chemical reaction, by application of pressure or by some combination thereof, becomes a solid which is hard and rigid. The solid state is permanent and irreversible. That a polymer is a thermoset is a property of the polymer. Not all polymers have that property.
The words cure and curing, when used in connection with a thermosetting resin, are employed to specifically refer to the process by which a thermoset is irreversibly converted to a solid material which is permanently hard and rigid. The curing process, which, as stated, proceeds by application of heat, by chemical reaction, by application of pressure or some combination thereof, causes the molecular chains of the polymer to react at chemically active sites of the polymer to form a cross-linked structure whose molecular weight and melting point is greater than that of the original thermoset. Heating of the formed permanently hard and rigid material after completion of the curing process will not cause the material to soften or melt to thereby enable it to be reformed. The material will decompose before melting. The formed solid material is said to be infusible.
A thermoplastic resin is a polymer which, upon application of heat, melts and/or softens and can be formed or molded. Upon cooling the polymer solidifies, becomes rigid and retains the molded shape. It can be reheated and again be melted and/or softened. The formed solid material is not a permanent condition. It can be reversed. The formed solid is said to be fusible.
The word phenolic is a term employed in the art to generally designate a family of synthetic polymers made by the reaction of phenol and an aldehyde, such as formaldehyde. Synthetic polymers thus generally designated by the term phenolic include a number of specific reaction products which include one-step phenolic resins (A-stage/resole resins) and novolac resins. The term resite designates a thermoset produced from a resole. (In this regard note Graham et al U.S. Pat. No. 3,929,191 and Underdown et al U.S. Pat. No. 4,443,347.) A phenolic resin can be classed as either a thermoset or a thermoplastic depending on its initial designation as a one-step phenolic resin, a novolac resin, or a resite.
Formaldehyde can react with phenol at three different sites (two ortho sites and one para site) on the phenol molecule. The reaction can be conducted in an acid environment or in an alkaline environment. Whether the synthetic polymer product of the reaction is classed as a thermoset or a thermoplastic is a function of the mole ratio of formaldehyde to phenol in the reaction.
It is known that base catalyzed phenol formaldehyde resins are made with a formaldehyde to phenol ratio of greater than one (usually around 1.5 to 1). It is believed that phenol-formaldehyde resins made with a formaldehyde to phenol ratio of greater than one are usually a thick reddish-brown liquid which will begin to cross link at a temperature of about 120°C. to produce a highly cross linked solid. Phenol-formaldehyde resins made with a formaldehyde to phenol ratio of greater than one do not require addition of a cross linking agent to cure.
The acid catalyzed reaction between phenol and formaldehyde involves a mole ratio of formaldehyde to phenol of less than one. The reaction product is referred to as a novolac. A novolac, due to the low molar ratio of aldehyde to phenol, will not completely cure without addition of a cross linking agent. Accordingly, a novolac is a thermoplastic synthetic polymer. It is fusible and does not become infusible until addition of an appropriate cross linking agent to complete the curing process. A novolac is generally alcohol soluble. Cross linking agents include hexamethylenetetramine and paraformaldehyde which react with the novalac at temperatures in the range of from about 200 to about 400° F. for conversion to cured, cross-linked structures. Such heating can proceed downhole in the presence of formation water.
The term resole is further defined in Underdown et al U.S. Pat. No. 4,443,347 in a way which is consistent with the definition provided in Graham et al U.S. Pat. No. 3,929,191. According to Underdown et al, a resole is a phenolic resin product of the condensation of a phenol with an aldehyde under alkaline conditions, wherein the condensation is stopped at an early stage. The resulting partial condensation is capable of further condensation to an infusible or thermoset condition.
Resite is the term employed to describe the infusible product produced by the further condensation of a resole.
An epoxy is a synthetic polymer commonly produced by the reaction of epichlorohydrin and bisphenol-A. The reaction product is a thermoset which cures when mixed with a hardening agent. The heat and chemical resistance of the cured thermoset can be improved by heating the cured thermoset to a value above room temperature, but heating beyond room temperature is not required in order to effect a complete cure. The hardening agent employed to cure the epoxy can be an amine, R—NH2, such as diethylene triamine (DETA) and triethylene tetramine (TETA).
Graham et al in U.S. Pat. No. 3,929,191 disclose the treatment of subterranean formations employing particulate solids coated with a synthetic resin. The synthetic resin disclosed in the 100 patent, is known by several names including A-stage resin, resole and one-step resin. This 101 resin is an alkaline-catalyzed, thermosetting phenol-formaldehyde resin consisting primarily of partially condensed phenol alcohols. The resin is soluble in certain solvents, such as alcohols and ketones, is fusible and is curable by application of heat alone without need of an additional catalyst or a curing agent. According to Graham et al, the mole ratio of aldehyde (formaldehyde) to phenol is in the range of from about 1 to 1 to about 1.5 to 1. Further, according to Graham et al, the polymerization reaction can be stopped by cooling the mass to form a solid, partially polymerized thermoset which is soluble in organic solvents and is fusible. Graham et al state, that the partially polymerized thermoset, in solid powdered form, can be dissolved in a suitable solvent, placed on a particulate solid where the thermoset, subsequent to evaporation of the solvent, remains as a coating. Thereafter, the coated particulate can be heated, for example, in a subsurface formation, and the resin, being fusible, is eventually cured to an insoluble, infusible cross linked state. It is understood that heating of the coated particulate in a subsurface formation can be expected to proceed in the presence of water naturally present in the formation. Accordingly, the invention in U.S. Pat. No. 3,929,191 features the placement of a curable resole resin on a particulate solid without the use of heat as a step in the process, but it is evident that heat is employed to initiate the polymerization reaction (the curing) before the resin is applied to the substrate.
Graham et al in U.S. Pat. No. 4,585,064 provide an extensive list of particulate solids employed in oil field service operations. These materials, in selected size ranges, have been used in oil field operations as naturally occurring, and also have been used in oil field operations after having been first coated with one or more layers of various different resinous materials, such as epoxy resins, resole resins and novolac resins. The patent also discloses a list of resins employed to coat particulate solids, and still further discloses a method of placing two layers of resin materials on a substrate.
Underdown et al in U.S. Pat. No. 4,443,347 define the terms resole, novolac, phenolic and cured as those terms are employed herein.
Some methods employed to coat particulate solids with a resinous material have featured the separate step of heating the particulate material to an elevated temperature in the range of from about 100 to about 600 degrees Fahrenheit, followed by uniformly mixing the heated particulate material with a fusible resinous material to coat the material and thereafter cooling the coated material to terminate the curing reaction which was initiated by the heated particulate solids.